This invention relates to an induction heater for floating zone melting of a crystalline semiconductor rod, and it relates, more particularly, to an induction heater comprising a flat coil and a cylindrical supplementary heater of a material having good electrical conductivity connected parallel with the flat coil.
An induction heater of the mentioned kind is described, for 10 example, in German patent document DE-AS No. 18 02 524. The supplementary heater consists of a cylinder, in which the flat coil is arranged in such a way that the cylinder projects on both sides beyond the flat coil, for example, equally. The part of the cylinder facing the polycrystalline supply rod serves as pre-heater and the part of the cylinder facing the monocrystalline semiconductor rod as an afterheater. The purpose of the supplementary heater was thought to lie in maintaining the temperature gradient on both sides of the melting zone in the adjacent solid rod parts of the semiconductor rod low.
Experiments with the described induction heater, however, have shown that the pre-heater produces unfavorable results. In the presence of the pre-heater, melting of the supply rods with respect to a simple flat coil takes place earlier. Depending on the quality of the polycrystalline silicon in this configuration, non-melting lugs directed axially form especially at the edge of the rod which are not melted by the flat coil. The drawing process, therefore, must be interrupted at this point, which renders the entire rod useless.